1. Field of the Invention
This invention relates to the field of dental restorative materials and adhesives. More particularly, it relates to the control of chemical reactions in chemically initiated dental restorative materials.
2. Description of the Prior Art
Dental polymeric restorative materials and adhesives are cured by the addition of very small amounts (e.g., 0.01 to 5.0 weight percent) of catalysts and co-initiators. See Bowen U.S. Pat. Nos. 5,270,351; 4,659,751; 4,588,756; 4,521,550; 4,514,527 and allowed U.S. patent application Ser. No. 07/791,999, filed Nov. 14, 1991. Most initiating systems consist of two active components; a free-radical initiator such as benzoyl peroxide or camphorquinone and a free radical-generating co-initiator consisting of a secondary or preferably tertiary aromatic amine or an aliphatic amine. Combining the peroxide and amine or quinone and amine, plus exposure to light, results in the generation of radicals that causes the polymerization of the adhesive, restorative material, and/or other monomer(s). The peroxide and amine components are generally separated during storage to prevent premature polymerization. This separation is done by mixing the small fractions of each of the coinitiators with separately stored portions of the resin or other ingredients. These two separate portions are then combined immediately before use to initiate the free radical-polymerization reaction under controlled conditions.
In some resin systems, the storage stability of the initiators is limited by exposure to oxygen or elevated temperatures. Solutions of certain amine compounds used in dental adhesives, such as NPG (N-phenylglycine), are known to degrade within a few days when exposed to air. Salt solutions of these compounds, such as the magnesium or sodium salt in acetone, are relatively stable for a number of months when exposed to air. The solid salt forms, however, are the most stable of all and can be stored exposed to air for years without adversely affecting them.
Several approaches have been used to store and deliver oxygen-sensitive adhesive materials. Products using NPG have been packaged in sealed glass syringe capules under argon to prevent oxygen contamination and exposure. Products using NPG-GMA (N-phenylglycine glycidylmethacrylate) have been packaged under inert gas in sealed glass ampules to prevent oxygen exposure. Both methods are expensive from the standpoint of manufacture and inconvenient for the operator to dispense and use. Other products deliver the amine as an acetone solution of the sodium or magnesium salt of NTG-GMA (N-tolylglycinc glycidylmethacrylate). The solutions have a shelf life of several months at room temperature that can be extended to over one year if refrigerated. These solutions, however, must still be dispensed and mixed with a corresponding acidic monomer, or monomer containing a coinitiator, to initiate polymerization.
Better approaches to the difficulties associated with the procedures discussed above have not been forthcoming. In the mid-1970's, Teledyne Dental Products Company of Chicago produced a dental restorative kit, sold under the trademark "COMPODENT", that had a paper mixing pad saturated with amine initiator. The product had a brief market life of about a year because problems developed in achieving proper proportioning of the initiators and adequate mixing using the system with viscous composites.